1. Field of the Invention
This invention relates to improvements in the method and means for pretreating a ferrous base metal strip in a metallic coating line, and more particularly to a method and means for maintaining a non-oxidizing atmosphere at positive pressure within the strip preparation furnace of the metallic coating line during line stops.
2. Description of the Prior Art
The method and means of the present invention are applicable to various types of metallic coating lines. For purposes of an exemplary showing, however, the present invention will be described in this application to a hot dip metallic coating line of the general type taught in U.S. Pat. No. 3,936,543.
In recent years, pretreatment or preparation of a ferrous base metal strip, prior to dipping in a bath of molten coating metal, has been carried out by the so-called Selas process (a detailed description of which may be found in U.S. Pat. No. 3,320,085, issued May 16, 1967 to C. A. Turner, Jr.), or a modification thereof. This process contemplates a direct fired furnace wherein oils and foreign materials are removed from the surfaces of the ferrous base metal strip, a heating furnace (having a hydrogen-nitrogen atmosphere) wherein the strip is brought to the desired maximum temperature to achieve the required final characteristics of the base metal strip and one or more cooling chambers wherein the ferrous base metal strip is brought down to a temperature compatable with the bath of molten coating metal into which it is to be immersed. The direct fired furnace, controlled atmosphere heating furnace and cooling chamber or chambers are connected in sealed relation so that the entire preparation furnace of the line can be operated above atmospheric pressure by controlling the discharge rate of the combustion products in the direct fired furnace. While the direct fired furnace controlled atmosphere heating furnace and one or more cooling chambers may be horizontally oriented with the strip passing therethrough on appropriately arranged support rolls, the more common presentday arrangement contemplates a vertically oriented direct fired furnace, a vertically oriented controlled atmosphere heating furnace and one or more vertically oriented cooling chambers, the strip passing therethrough in vertical flights, guided by rolls at the upper and lower ends of the direct fired furnace, controlled atmosphere heating furnace and one or more cooling chambers. From the last cooling chamber, the strip is conducted through a snout, the end of which is submerged beneath the surface of the molten coating metal in the coating pot so that throughout its passage through the preparation furnace of the line and into the bath of molten coating metal the strip will travel through controlled atmospheres. An example of a coating line preparation furance of the type contemplated, is taught and illustrated in U.S. Pat. No. 3,837,790.
Heretofore, during line stops the exhaust fan of the direct fired furnace would withdraw gases from the preparation furnace of the metallic coating line. With the direct fired furnace shut down, these gases comprise primarily hydrogen and nitrogen from the controlled atmosphere heating furnace and one or more cooling chambers. The loss of these gases not only constitutes an expense, but also results in a loss of pressure within the entire preparation furnace, enabling oxygen from the ambient atmosphere to enter into the elements of the preparation furnace through openings and seams therein. This, in turn, will result in oxidation and scaling of that part of the strip located within the preparation furance of the coating line. When this portion of the strip is caused to pass through the bath upon start up, coating defects will appear on the strip by virtue of the oxidation and scaling. The oxidation and scaling may also produce bath contamination and damage to the pot rolls. The presence of oxygen in the snout may result in the collection of impurities within the snout, and dross build-up when the molten coating metal is aluminum.
By providing a method and means for sealing the direct fired furnace from the conduit leading to its exhaust fan and the air dilution opening in that conduit, and upon the addition of excess nitrogen within the preparation furnace portion of the coating line, the entrance of oxygen from the ambient atmosphere about the direct fired furnace, controlled atmosphere heating furnace, one or more cooling chambers and snout will be prevented. Heat loss within these elements will also be reduced, resulting in a reduction in the amount of warmer coils used to return the line to prime production after a lengthy line stop.
The advent of the Selas-type furnace treatment process brought the benefit of being able to maintain a positive pressure throughout the preparation furnace while the coating line is operating. The method and means of the present invention extend that same benefit of maintaining a positive pressure to down-time as well as operating time.